General Description: Two of the CLKDLL-related properties can be put on the BUFGDLL element, and they should control the behavior of the underlying CLKDLL simprim: these are DUTY_CYCLE_CORRECTION and the new FACTORY_JF. If these are specified in the UCF file, the new values will not be pushed down to the underlying CLKDLL, and the simprim won't see them, so the simulation will not correctly simulate these behaviors.
Solution
To workaround this issue, use the following syntax in the ucf file: